First you must know exactly what a search engine is: A search engine is a user interface that by entering a search term displays a set of relevant results from and index (database) on dynamically generated pages giving links to websites of that subject matter. The higher the link should indicate the higher relevancy to what the user is looking for. Using this method websites are ranked. The search engine uses bots and spiders to search through the Internet for other websites and new pages to add to its index that it also finds relevant. The higher a website within the rankings ultimately should deem how relevant it is to what the user is looking for.
Research implies that approximately 48% of all search requests for key words are entered through the Google search engine. Yahoo follows secondly with about 22% of search requests. Microsoft searches are a distant third with about 11% of all requests.
There is increased exposure to this search engines with more and more people setting them as their home pages and downloading toolbars with search boxes linking directly into the search engine site displaying query results.
Search engine optimisation is a set of techniques employed to improve the ranking of a website or indexed web page. The purpose from a marketing perspective is to get more people to visit that website through the higher ranked web pages.